It is well known in the art that processes for the preparation of a copolymer of carbon monoxide and an olefinic monomer can involve the copolymerization of the monomers in the presence of a catalyst composition containing a Group VIII metal and a bidentate ligand of the general formula R.sup.1 R.sup.2 M.sup.1 --R--M.sup.2 R.sup.3 R.sup.4, wherein M.sup.1 and M.sup.2 independently represent a phosphorus, nitrogen, arsenic or antimony atom, R.sup.1, R.sup.2, R.sup.3 and R.sup.4 independently represent a nonsubstituted hydrocarbyl group and R represents a bivalent organic bridging group containing at least 1 carbon atom in the bridge, in a diluent in which the prepared copolymer forms a suspension. EP-A-213671 presents such a process. The copolymers prepared are aliphatic copolymers wherein the monomer units originating from carbon monoxide and the monomer units originating from the olefinically unsaturated compounds occur in alternating or substantially alternating order.
The economy of such processes can be improved by increasing the reactor productivity. The productivity of a polymerization reactor may be defined as the quantity of copolymer which can be prepared within a given period of time using a given reactor volume. The reactor volume is normally in a direct relation with the suspension volume or the volume of the liquid phase. In this document the liquid phase volume is meant to include the volumes of a liquid diluent and any monomer present in the liquid phase at the conditions of the copolymerization, but it excludes the volume of the copolymer suspended in the liquid phase.
The reactor productivity may be increased by raising the temperature or the pressure so that the specific polymerization rate, i.e. the quantity of copolymer produced per mole Group VIII metal per hour, is increased. However, by raising the temperature or the pressure structural features of the prepared copolymer such as the molecular weight may also change. This is not necessarily desired.
The quantity of the Group VIII metal present in the reaction mixture has typically been below 0.1 mmole per liter liquid phase, most frequently in the range of 0.01-0.05 mmole per liter liquid phase. EP-95200719.3 comprises an example of a copolymerization of carbon monoxide with olefinically unsaturated compounds in dichloromethane as a liquid diluent using a catalyst composition based on palladium as a Group VIII metal, 1,3-bisbis(2-methoxyphenyl)phosphino!propane as a bidentate ligand of the general formula R.sup.5 R.sup.6 M.sup.1 --R--M.sup.2 R.sup.7 R.sup.8 and, in addition, tris(perfuorophenyl)borane as an example of a hydrocarbyl boron compound. Prior to the copolymerization a copolymer was suspended in the diluent. The catalyst composition was used in a quantity which contains 0.025 mmoles Group VIII metal and a liquid phase volume was applied which is obtainable by combining in a 300 ml autoclave at 80.degree. C. 110 ml dichloromethane, 25 g propene and such a quantity of a carbon monoxide/-ethene 1:1 molar mixture that a 5.0 MPa pressure is obtained.
In principle, an increase in the reactor productivity may be accomplished by raising the concentration of the catalyst present in the polymerization mixture so that more copolymer may be produced in the same volume and within the same time. However, Applicant has found that, when using as the bidentate ligand a compound of the general formula R.sup.1 R.sup.2 M.sup.1 --R--M.sup.2 R.sup.3 R.sup.4, an increase of the catalyst concentration to a higher value, i.e. outside the range of 0.01-0.05 mmole Group VIII metal per liter liquid phase, leads to a decreased specific polymerization rate and to a decreased molecular weight of the copolymer. These results are not satisfactory.
It has now been found that an increase in the concentration of the catalyst in the polymerization mixture does not lead to a change in the specific polymerization rate if prior to the polymerization a previously prepared copolymer is suspended in the diluent and if as a catalyst component a bidentate ligand is used which comprises a polar substituted hydrocarbyl group at the dentate groups. In this case, the productivity increases in proportion with the catalyst concentration. It is also advantageous and surprising that under these circumstances there is no change in the molecular weight of the copolymer prepared.